Duckbill-shaped elastomer check valves are sometimes installed on stor
m-water or sewage outfall ports to prevent backflow or seawater intrus
ion. Unlike fixed diameter ports, the jet velocity varies nonlinearly
with port discharge flow an analysis of all available experimental dat
a on the head-discharge characteristics of check valves shows that the
collective data can be effectively correlated with the parameters of
the approach flow in the discharge port, Two dimensionless relations a
re obtained for the effective percentage jet opening area and the valv
e head loss. The relations are derived from data for nominal port diam
eters in the range of 100-305 mm (4-12 in.), and appear to hold genera
lly for elastomer check valves. The head-discharge relations can be us
ed to determine the jet discharge velocity and opening area that are n
eeded as input to numerical plume models for environmental impact asse
ssment and outfall design. The results can also be conveniently incorp
orated into a diffuser hydraulics code. In general, the calculations s
how that the use of duckbill check valves leads to a more uniform port
discharge distribution, higher jet velocities, and higher energy head
requirements.